linux-kernel - Re: [RFC PATCH 1/4] hazptr: Add initial implementation of hazard pointers

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Message-ID: <1b04e35c-8994-48d9-907a-966bb4dfabaf@huaweicloud.com>
Date: Fri, 27 Sep 2024 21:12:09 +0200
From: Jonas Oberhauser <jonas.oberhauser@...weicloud.com>
To: Linus Torvalds <torvalds@...ux-foundation.org>,
 Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
Cc: Boqun Feng <boqun.feng@...il.com>, linux-kernel@...r.kernel.org,
 rcu@...r.kernel.org, linux-mm@...ck.org, lkmm@...ts.linux.dev,
 "Paul E. McKenney" <paulmck@...nel.org>,
 Frederic Weisbecker <frederic@...nel.org>,
 Neeraj Upadhyay <neeraj.upadhyay@...nel.org>,
 Joel Fernandes <joel@...lfernandes.org>,
 Josh Triplett <josh@...htriplett.org>,
 "Uladzislau Rezki (Sony)" <urezki@...il.com>, rostedt <rostedt@...dmis.org>,
 Lai Jiangshan <jiangshanlai@...il.com>, Zqiang <qiang.zhang1211@...il.com>,
 Peter Zijlstra <peterz@...radead.org>, Ingo Molnar <mingo@...hat.com>,
 Will Deacon <will@...nel.org>, Waiman Long <longman@...hat.com>,
 Mark Rutland <mark.rutland@....com>, Thomas Gleixner <tglx@...utronix.de>,
 Kent Overstreet <kent.overstreet@...il.com>, Vlastimil Babka
 <vbabka@...e.cz>, maged.michael@...il.com,
 Neeraj Upadhyay <neeraj.upadhyay@....com>,
 Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Subject: Re: [RFC PATCH 1/4] hazptr: Add initial implementation of hazard
 pointers

Am 9/27/2024 um 8:13 PM schrieb Linus Torvalds:

> Because even hiding the value one from the compiler will mean that it
> can't use the comparison to decide that the originals are equal, even
> if one of them is unhidden.
> 
> No?
> 
>                Linus

I think it depends on which one you hide.

If you do

  z = b;
  hide(z);
  if (a==z) { *b; }

then it will be fine, because it knows a==z but nothing about the 
relation of b with a or z.

But for

  z = a;
  hide(z);
  if (z==b) { *b; }

then it would still know that b == z, and could replace *b with *z 
(which really is *a).

Best wishes,
   jonas